In several data storage systems, such as hard disk drives, the recording head (or heads) is positioned over the disk surface by a rotary or linear actuator. The actuator is moved, or positioned, by a motor. With disk drive systems, a voice coil motor moves the actuator. A voice coil motor is, in simple terms, a coil between two magnets. To move the actuator, a current is applied to the coil to induce a force, and this force moves the actuator.
A recording head reads data from, and/or writes data to, the disk. So positioning the recording head accurately is very important in data storage systems. Typically, the recording head is located at one end of the actuator. But accurate positioning of the recording head can be difficult due to the transient motion created when the actuator and head are moved. Since the actuator is similar to a long beam, moving it causes the actuator to oscillate, or resonate. And this in turn causes the recording head to oscillate. This oscillation, or resonance, makes accurate positioning of the recording head difficult.
One conventional method for compensating for the oscillations of the recording head is to determine the structure of the actuator and recording head in detail. Once known, the movement of the actuator and head can be determined and the resonant frequencies calculated. A control system can then be designed to cause the motor to move in a direction contrary to the transient motion, thereby minimizing, or canceling, the resonance frequencies. A limitation to this method however, lies in the fact that the actuator and recording head oscillate in several directions, thus creating a high number of differing resonance frequency states. Unfortunately, this causes the control system to be complex, which in turn causes the cost of developing and manufacturing such a control system to be high.
To overcome the limitations of the one stage control system described above, dual stage control systems have been proposed. In a dual stage control system, a microactuator is combined with a coarse actuator. A coarse actuator is usually a conventional actuator, while a microactuator typically moves the recording head only. In this type of control system, the actuator is used for the coarse positioning of the recording head, while the microactuator is used for high speed, high accuracy positioning of the recording head.
The design of dual stage control systems, however, is much more complicated than the single stage control system. Anticipating and effectively compensating for the high frequency transient motion of the actuator and recording head is challenging. The undesired transient motion is usually generated by the actuator, suspension, and gimball resonances
A typical method of compensation utilizes transfer function models of the coarse actuator and the microactuator. One conventional control scheme for compensating the coarse actuators undesired transients in dual stage control systems in shown in the block diagram of FIG. 1. Blocks 108, 110, 114 and 130 in FIG. 1 represent mathematical functions that can be implemented in software and/or hardware.
The control scheme 100 includes a controller 102, a coarse actuator 104, and a microactuator 106. The coarse actuator 104 is comprised of a Vnom block 108 and a Vres block 110. Vnom is usually an ideal frictionless model, or transfer function, of the coarse actuator 104. For voice coil motor actuators, such as the ones found in hard disk drives, the most frequently used transfer function for the nominal model is Vnom(s)=1/s2, where s is the Laplace transform operator. The resonant portion of the coarse actuator, Vres, is usually determined via finite element analysis of the mechanical structure and/or frequency domain measurements.
The controller 102 is comprised of a control block 112 and a filter 114. Controller 102 is typically a state variable feedback controller. Certain types of microactuators 106 allow explicit measurement of their displacement, and this measurement may be fed back to the controller 102 via line 115.
The position of the recording head relative to the storage medium is measured by means of special servo marks written on, or formed in, the storage medium. The current position of the recording head (y) is fed back as input into controller 102 via line 116. A previously presented position for the recording head (a previously presented specific location where the head is to be moved to) is input into the controller via line 118. Control block 112 then generates two signals, one on line 120 and the other on line 122. The signal on line 120 is input into anti-resonance filter 114 to generate a previously presented signal, uv, on line 123. Filter 114 is used to compensate for the resonance of the coarse actuator, and is described in greater detail below.
The signal uv is input into the coarse actuator 104, where the transfer functions Vnom and Vres are applied to the signal uv to generate a signal yv on line 124. In hard disk drive systems, uv is the current used to move the coarse actuator, and the signal yv represents the positioning motion of the coarse actuator. The signal um on line 122 is input into microactuator 106 and causes the microactuator 106 to move. The microactuator 106 then generates a signal ym on line 126, which represents the positioning motion of the microactuator 106. The position of the recording head (y) on line 128 is the sum of the coarse actuator's motion yv and the microactuator's motion ym, as shown in block 130.
To simplify the controller structure and the design procedure, the controller 102 is usually designed for the nominal portion of the coarse actuator (Vnom), neglecting the resonances, Vres. To minimize the influence the resonances have on the system, a cascade anti-resonance filter 114 is applied. The anti-resonance filter 114 approximates the inverse of the resonance model, i.e. C(s)=1/Vres(s). Thus, the resonant portion of the coarse actuator is cancelled out by inverting the resonance transfer function, Vres.
One limitation to this method, however, is that in many cases Vres contains unstable (right half s-plane) zeros. When an unstable zero is inverted, an unstable pole is created. This results in an unstable pole-zero cancellation, which makes the system unstable (i.e., can not control the motion), and may result in complete failure of the control system.